Alpha Centauri (Star)/meta

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When comparing real-astronomical star positions to the Traveller Map of Charted Space, it needs to be observed that the axes of the Charted Space Map appear to be tilted almost exactly 45o counter-clockwise from the correct position (i.e. "true" Coreward actually lies parallel to the Lesser Rift). If one makes that adjustment, about 70% of the named stars will fall roughly along their correct bearing. However, there are some exceptions in which the star in question lies in roughly the correct position relative to the Charted Space Map without the need to rotate the coordinate axes.

In general, if one is assigning the position of a Real-Universe star to a Traveller Map hex, it should be acceptable if:

1) The star's longitude angle (θ) is within a 45o bearing-arc between the "true" coordinate axes and the Traveller Charted Space Map coordinate axes, and
2) The star's assigned distance from Terra on the hexmap falls somewhere between its true distance R in parsecs, and its 2D-projection onto the flat map as seen from above, found by Dproj = {R * cos(ɸ)}, where (ɸ) is the latitude angle. If desired, the distance above/below the plane can be determined by Z = {R * sin(ɸ)}.

Note on Alpha Centauri: Alpha Centauri is one of the best illustrations of the axis-problem noted above. Note that Alpha Centauri actually lies at galactic longitude coordinates almost due Coretrailing (315o) from Earth, but is shown as due Trailing (270o) on the Map of Charted Space. This is almost exactly a 45o rotation.


Alpha Centauri Bc

In March 2015, a transit event of Alpha Centauri B was witnessed possibly corresponding to a planetary body. This planet would most likely orbit Alpha Centauri B with an orbital period of 20.4 days or less and a likely eccentricity of 0.24 or less. If confirmed, this planet would be called Alpha Centauri Bc. It would be far too close to Alpha Centauri B to harbour life.

Additional planets

Additional planets may exist in the Alpha Centauri system, and could orbit Alpha Centauri A or Alpha Centauri B individually, or be on large orbits around the binary Alpha Centauri AB system. All observational studies have so far failed to find any evidence for brown dwarfs or gas giants.

To be in the star's habitable zone, any suspected planet around Alpha Centauri A would have to be placed about 1.25AU away in order for planetary temperatures and conditions for liquid water to exist. For the slightly less luminous and cooler Alpha Centauri B, the habitable zone would lie closer at about 0.7AU. Computer simulations in 2009 showed that a planet might have been able to form near the inner edge of Alpha Centauri B's habitable zone, which extends from 0.5 to 0.9 AU from the star. Bodies around A would be able to orbit at slightly farther distances due to A's stronger gravity. In addition, the lack of any brown dwarfs or gas giants in close orbits around A or B make the likelihood of terrestrial planets greater than otherwise. Planets of more than 4 M out to the distance of the habitable zone of the star (an orbital period of ~200 days) have been ruled out by astronomers.

A sub-millimetre source detected in 2014, nominally tagged as "αCen D", may be a substantial companion, or an extreme Trans-Neptunian Object (TNO).

There is no direct evidence yet for the existence of an Oort cloud around Alpha Centauri AB, and theoretically this may have been totally destroyed during the system's formation.

Searches for companions orbiting Proxima Centauri have been unsuccessful, ruling out the presence of both brown dwarfs and supermassive planets. Surveys have also ruled out the presence of super-Earths within the star's habitable zone. Measurements appear to rule out any gas giant companions with an orbital period of 2−12 years. The habitable zone of Proxima Centauri lies at about 0.023–0.054 AU from the star, and a world located here would have an orbital period of 3.6–14 days. A planet orbiting within this zone would experience tidal locking to the star, and most of the surface would experience either day or night perpetually, except in a thin circumpolar band of twilight. However, the presence of an atmosphere could serve to redistribute the energy from the star-lit side to the far side of the planet.

  • UPDATE (August 2016): The Earth-like exoplanet Proxima Centauri b (also called Proxima b) has been discovered orbiting within the habitable zone of Proxima Centauri. The planet is at a distance of roughly 0.05 AU, with an estimated mass of 1.3 M, and an orbital period of approximately 11.2 standard days. The equilibrium temperature of Proxima Centauri b is estimated to be within the range where water could exist as liquid on its surface, thus placing it within the habitable zone of Proxima Centauri.
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